Northern Appalachian Orogen Research Articles

Rapid transition from the salinic to Acadian orogenic cycles in the northern Appalachian Orogen: Evidence from northern New Brunswick, Canada

Geochronological data from volcanic rocks aid in reconstructing the Silurian–Devonian evolution of the northern Appalachians of New Brunswick in the context of Salinic (Silurian) and Acadian (Devonian) orogenesis. Late Silurian to Early Devonian sedimentation, volcanism and deformation in northern New Brunswick is complex, and characterized by transgressive-regressive sedimentary cycles, local disconformities and angular unconformities, and two stages (ca. 422–419 Ma, and ca. 417–407 Ma) of typically bimodal, within-plate magmatism. These events overlapped the end of the Salinic orogenic cycle, related to the ca. 430 Ma collision of the Ganderian passive margin with composite Laurentia, and the beginning of the Acadian cycle, which is associated with subsequent (ca. 424–422 Ma) collision and underthrusting of Avalonian crust beneath composite Laurentia (now including the Gander margin). Following Ganderia−Laurentia collision, the earlier (Pridolian) stage of within-plate magmatism is interpreted to result from ca. 425 Ma detachment of the subducting Tetagouche backarc lithospheric slab and is linked to the Salinic cycle, along with local, pre-Acadian (Salinic C) deformation of Ludlow−Pridoli rocks. Break-off of the Tetagouche slab also led to rapid uplift and subsequent extensional collapse of the Salinic metamorphic core, resulting in latest Silurian-earliest Devonian D3 vertical shortening and flat belt formation in the Brunswick subduction complex. Precise ID-TIMS U-Pb (zircon) dating of volcanic rocks reveals the existence of a 2.2 myr hiatus in the stratigraphic record, corresponding to an interpreted latest Silurian–earliest Devonian erosional surface between volcanic rocks identified with the Salinic cycle (Dickie Cove Group and lower part of Tobique Group) and those associated with the Acadian cycle (Dalhousie Group). The disconformity coincides with a widespread marine regression recorded by roughly coeval deposition of shallow-water to intertidal redbeds across the northern Appalachians. Marine regression is closely followed by deposition of Lochkovian deep marine sedimentary rocks (turbidites) in the Seboomook foredeep. This regressive-transgressive sequence immediately precedes arrival of the northwest-migrating Acadian deformation front, and is interpreted as evidence of a migrating Acadian forebulge-foredeep system associated with flexural loading of Ganderian crust by the Acadian orogenic wedge situated farther southeast in southern New Brunswick and coastal Maine. Early Devonian volcanism and Acadian retro-arc foreland deformation are linked to flat-slab convergence of Avalonia after underthrusting of buoyant Avalonian continental crust began ca. 418 Ma. It is proposed that the latest Silurian−earliest Devonian disconformity and marine regression mark the beginning of the Acadian orogenic cycle, implying that, in northern New Brunswick, the effects of waning, late Silurian (Salinic) deformation temporally overlapped the early, far-field effects of Acadian orogenesis.

The Litchfield Pluton in South-Central Maine: Carboniferous Alkalic Magmatism in northern New England, USA

The Litchfield pluton is a poorly exposed 7 km2 composite alkalic intrusive complex that cuts previously deformed and metamorphosed Silurian turbidites in south-central Maine. The pluton includes a variety of alkaline syenites, including the type locality of “litchfieldite”, a coarse-grained cancrinite, sodalite, and lepidomelane bearing nepheline syenite first recognized over 150 years ago and common in many petrologic collections. A new U-Pb zircon age of 321 ± 2 Ma from the nepheline syenite is interpreted to represent the crystallization age of the plutonic complex. A new biotite 40Ar/39Ar age of 239 ± 1 Ma from the syenite is similar to previously published mica ages from the surrounding country rocks and dates the time of regional cooling in the area below ~ 300°C. Whole rock geochemical analyses from rocks of the Litchfield pluton are compatible with strongly alkaline A-type granitoid rocks that formed in a within plate or continental rift tectonic setting. The age and geochemical characteristics of the alkalic igneous rocks near Litchfield are consistent with a model that invokes the generation of a small volume of alkalic magma beneath south-central Maine during a period of Carboniferous transcurrent tectonism in the northern Appalachian orogen.

Late Paleozoic strike-slip faults in Maritime Canada and their role in the reconfiguration of the northern Appalachian orogen

Major late Paleozoic faults, many with documented strike-slip motion, have dissected the Ordovician-Devonian Appalachian orogen in the Maritime Provinces of Atlantic Canada. Activity alternated between east-west faults (Minas trend) and NE-SW faults (Appalachian trend). NW-SE faults (Canso trend) were probably conjugate to Minas-trend faults. Major dextral movement, on faults with Appalachian trend, in total between 200 and 300 km, began in the Late Devonian. This movement initiated the Maritimes Basin in a transtensional environment at a releasing bend formed around a promontory in the Laurentian margin and thinned the crust, accounting for the major subsidence of the basin. Appalachian-trend strike slip continued in the Mississippian but was accompanied by major movement on E-W Minas-trend faults culminating around the Mississippian-Pennsylvanian boundary, juxtaposing the Meguma and Avalon terranes of the Appalachians close to their present-day configuration. However, strike slip continued during the Pennsylvanian-Permian interval resulting in transpressional deformation that reactivated and inverted earlier extensional faults. A final major episode of transtension, mainly sinistral, occurred during the Mesozoic opening of the Atlantic Ocean. Restoration of movements on these faults, amounting to several hundred kilometers of slip, explains anomalies in the present-day distribution of terranes amalgamated during early Paleozoic Appalachian tectonism. In the restored geometry, the Nashoba and Ellsworth terranes of Ganderia are adjacent to one another, and the Meguma terrane lies clearly outboard of Avalonia. A restored post-Acadian paleogeography, not the present-day geometry of the orogen, should be used as a basis for reconstructions of its earlier Paleozoic history.

The Turner Mountain syenite, Maine, USA: geology, geochemistry, geochronology, petrogenesis, and post-orogenic exhumation

The Turner Mountain syenite is one of the few plutons located entirely within the Norumbega fault system in the northern Appalachian orogen. It is composed of texturally and mineralogically homogeneous biotite-amphibole syenite and is in faulted contact with mylonitic leucogranite and an unmetamorphosed redbed unit. It is intermediate in SiO2 content (58.7–65.1 wt%) and ultrapotassic (6.4–7.9 wt% K2O) with high K2O/Na2O ratios (2.75–4.15), yet is relatively primitive in terms of MgO (2.8–4.9 wt%), Ni (average 90.2 ppm), and Cr (average 210.1 ppm) contents. It has enriched large-ion-lithophile elements relative to high field strength elements, high contents of light rare-earth elements, and initial 87Sr/86Sr ratios (0.7038–0.7068) similar to that of OIB basalts. It differs petrologically and geochemically from the neighboring Lucerne-Deblois plutons but is similar to Lincoln syenite located 100 km to the southwest, also within the Norumbega system. Zircon U-Pb dating using LA-ICP-MS yields a weighted mean age of 410.5 ± 2.4 Ma, slightly younger than the Lincoln syenite (418 ± 1 Ma). Based on their distinctive geochemical signatures, both were probably products of Late Silurian-Early Devonian ultrapotassic magmatism related to Acadian subduction, generated by partial melting of a mantle wedge metasomatized by potassium-rich fluids during west-directed subduction. This unique magmatism could be attributed to decompressional melting during Late Silurian-Early Devonian slab break-off or delamination. Based on Sr-Nd isotopic compositions, the Turner Mountain syenite magma probably had more crustal interaction than that which formed the Lincoln syenite. The syenite was later exhumed tectonically during brittle reactivation of the Norumbega fault. The reactivation involved regional-scale, high-angle, southeast-over-northwest reverse faulting in a transpressional environment and occurred during the Late Devonian and through Mississippian to Permian.

The Appledore Island pluton of the Rye Complex, coastal New Hampshire and Maine, USA: geochronological and chemical evidence for the affinity of an enigmatic terrane

The Rye Complex of coastal New Hampshire and Maine is a peri-Gondwanan terrrane that up to now had an uncertain origin. An offshore portion of the complex, Appledore Island of the Isles of Shoals, hosts a mainly dioritic intrusion that yielded an U-Pb zircon age of 361.09 ± 0.14 Ma, allowing comparison of its geochemical characteristics with mafic rocks of similar age across the northern Appalachian orogen. The Appledore Island diorite has similar major, trace, and isotopic compositions as continental rift tholeiite in the Narragansett Basin in southern New England and in the Maritimes Basin of Canada. These intraplate volcanic rocks range from 375 to 330 Ma, bracketing the age of the Appledore Island diorite. Their intraplate tectonic setting reflects regional extension during the Late Devonian to Early Carboniferous which produced successor basins after the Acadian orogeny. The geochemical and age similarities of the Appledore Island diorite and the mafic rocks of the successor basins suggest that the Rye Complex is a basement fragment of a successor basin block. Further evidence of the identity of the Rye Complex is provided by the isotopic composition of intermingled, comagmatic granitic rocks associated with the Appledore Island diorite. The granite has a Ganderian isotopic signature, suggesting that the Rye Complex is a Ganderian basement block that was transposed by movement along the Norumbega Fault System to its position adjacent to the Merrimack Trough of New Hampshire and Maine.

Peri-Gondwanan terrane interactions recorded in the Cambrian–Ordovician detrital zircon geochronology of North Wales

Precambrian to Ordovician sedimentary basins of Wales display contrasting histories across major NE-striking fault systems. Northeast of the Menai Strait Fault System the Monian Supergroup is a deformed succession of mainly metaclastic rocks. Within the fault system the Arfon Basin contains sandstone and slate overlying Neoproterozoic volcanic rocks. South of the fault system, in the Harlech Dome of the Welsh Basin, a distinctive succession of clastic rocks has been correlated with those of the Meguma Terrane in the northern Appalachian orogen, together comprising the domain Megumia. A detrital zircon analysis from Cambrian sandstone in the Harlech Dome is consistent with this correlation. However, the Early Ordovician Dol-cyn-afon Formation higher in the succession and the overlying Late Ordovician Conway Castle Sandstone show a more diverse provenance, consistent with derivation from the Monian Supergroup. Cambrian sandstone from the Llanberis Slates Formation in the Arfon Basin shows a distinct provenance dominated by a Neoproterozoic source. None of the samples analyzed contains Laurentian detritus. These results suggest that the Welsh Basin was juxtaposed with the Monian Supergroup and its Precambrian substrate along the Menai Strait Fault System by the Tremadocian, and indicate that the Iapetus Ocean remained open at least until the Silurian. The Cambrian detrital zircon record from the Arfon Basin does not show clear similarity to the Monian Supergroup, nor to the Welsh Basin and adjacent Midland Platform, indicating that the basin was isolated from these sedimentary sources within the fault system. The juxtaposition of these terranes probably took place during strike-slip to transpressional tectonism close to the northern margin of Gondwana during the Early Ordovician.

Hydrothermal Dolomitization of Upper Ordovician Limestone, Central-East Canada: Fluid Flow in a Craton-Interior Wrench-Fault System Likely Driven by Distal Taconic Tectonism

Narrow (tens of meters) vertical bodies of dolomite replace a nonporous Upper Ordovician (lower Chatfieldian) limestone along the leading limb of an asymmetric faulted anticline well inboard (∼300 km) of the northern Appalachian orogen, central-east Canada. Patterns of geometry, texture, and geochemistry suggest that dolomitizing fluids were focused (possibly along microfractures) within a paleosinistral transpressive stress field generated with reactivation of an underlying Neoproterozoic fault system. Strike-slip failure of the developing anticline resulted in fracture- and fault-controlled fluid flow from which precipitated saddle dolomite. A paragenetic succession of Fe-poor planar-e to ferroan planar-s dolostone marks the peak phase of replacement dolomitization. A subsequent increase (20%–30%) in porosity created through local dissolution of relict limestone was partially occluded by ferroan planar dolomite that, geochemically, is similar to later fracture-fill saddle dolomite. Replacement dolomitization is associated with a slight rise in Sr-isotope ratios (to 0.71085) from Late Ordovician–early Silurian marine signatures. Isotope (C, O, Sr) signatures support influx and mixing of burial fluids that had interacted with local Mg-rich (gabbro, anorthosite, syenite) crystalline basement with a background fluid similar to Late Ordovician seawater or dissolved marine limestone. Dolomitization predated maximum burial in the Late Paleozoic. Fluid inclusion and isotope paleothermometry suggest that dolomitizing temperatures (100°–120°C) were 20°–30°C warmer than associated with prior limestone diagenesis. The dolomitized limestone is an archival record of structure, hydrology, and heat flux that best fits with Taconic tectonism in the latest Ordovician through earliest Silurian.

Infrastructure and provenance of Ganderia: Evidence from detrital zircon ages in the Brookville terrane, southern New Brunswick, Canada

The Brookville terrane in southern New Brunswick contains some of the oldest known sedimentary rocks of the Ganderian microcontinent of the northern Appalachian orogen. It includes the stromatolitic metacarbonate- and quartzite-dominated Ashburn Formation and metasiltstone-dominated Martinon Formation, in mylonitic contact with the Brookville Gneiss, an assemblage of low-pressure/high-temperature paragneiss and tonalitic orthogneiss. All of the metasedimentary units were intruded by the ca. 550–528Ma subduction-related Golden Grove Plutonic Suite. Analysis by LA-ICP-MS and TIMS of zircon grains from the Ashburn Formation quartzite yielded no <10% discordant ages younger than ca. 1200Ma, a predominance of Meso- and Paleoproterozoic ages back to ca. 2100Ma, and a few Neoarchaean ages. A sample from the metasiltstone matrix of a carbonate olistostrome in the Martinon Formation contains rounded grains with ages mainly between ca. 1000Ma and 2200Ma, indicating provenance similar to that of the Ashburn Formation. However, the sample also contains euhedral grains with ages averaging around 650Ma, indicating the maximum depositional age. A quartzite/calc-silicate sample from the Brookville paragneiss yielded detrital zircon grains with most ages between 1150Ma and 2000Ma, indicating provenance similar to that of the Ashburn and Martinon formations. An orthogneiss sample yielded euhedral (igneous) zircon grains with a concordia age of 615±4Ma, indicating an interval of continental margin subduction at least 60Ma older than that represented by the Golden Grove Plutonic Suite. The Ashburn and Martinon formations and Brookville paragneiss are evidence for development of a Late Cryogenian passive margin on the Proto-Andean-Caribbean edge of Amazonia after it had separated from Laurentia by ca. 650Ma to open the Puncoviscana Ocean. By 615Ma at least part of this margin had become a subduction zone which remained active during the Late Neoproterozoic-Early Cambrian, part of the Pampean arc system. Brookville and related terranes rifted away in the mid-Cambrian to form the foundation of Early Paleozoic Ganderia, which ultimately collided with composite Laurentia during the Salinic orogeny.

Lithostratigraphic, Hydrothermal, and Tectonic Setting of the Boundary Volcanogenic Massive Sulfide Deposit, Newfoundland Appalachians, Canada: Formation by Subseafloor Replacement in a Cambrian Rifted Arc

The Boundary volcanogenic massive sulfide (VMS) deposit (~ 0.5 Mt @ 3.5% Cu, 4% Zn, 1% Pb, 34.0 g/t Ag) in the Tally Pond Group, central Newfoundland, represents one of the best preserved, subseafloor-replacement style VMS deposits in the northern Appalachian orogen. The deposit is hosted within a Late Cambrian (~510 Ma) volcanic sequence consisting predominantly of rhyolitic flows and associated volcaniclastic rocks. Footwall strata are dominated by rhyolitic lapilli tuff, tuffs, lesser rhyolite flows, and in situ rhyolite breccias. The hanging wall consists of massive, quartz-bearing, flow-banded lobe and breccia facies rhyolite. The deposit occurs at and below the contact between these two units, and comprises pyrite, chalcopyrite, and lesser sphalerite (and other minerals). Massive mineralization contains abundant clasts of the surrounding host rocks, including chlorite-sericite-quartz–altered rhyolite lapilli and ash. Hydrothermally altered rocks consist of variably intense chlorite with lesser sericite and quartz. Chlorite alteration occurs in a discordant geometry, likely representing hydrothermal upflow zones, and chlorite-sericite-quartz occur as blankets that are parallel to the volcanic stratigraphy, likely representing alteration associated with replacement. The hanging-wall rhyolite flows also contain moderate to intense, pervasive quartz and sericite alteration. Both the hanging wall and footwall are characterized by strong Na2O-Sr depletions, K2O-MgO-Fe2O3-Ba enrichments, high alteration index values (e.g., Ba/Sr, chlorite-carbonate-pyrite index (CCPI), alteration index (AI)), and enrichments in base metals and volatile metals (e.g., Cu, Zn, Pb, Hg). The presence of abundant remnant wall-rock and host-rock clasts within the ore, intricate sulfide replacement of laminated porous tuff and sand dikes, replacement fronts in host lithofacies, and intense alteration in both the footwall and hanging wall (where the hanging wall is preserved) are all features consistent with formation of the bulk of the Boundary deposit via subseafloor replacement. The deposit likely formed as a result of cooling of metal-bearing hydrothermal fluids, mixing with ambient seawater and pore water-entrained seawater within the volcanic rocks at a permeability interface between young, unlithified, highly permeable footwall volcaniclastic rocks and relatively impermeable hanging-wall rhyolitic flows. In our model, this permeability boundary was an important feature that promoted subseafloor replacement within the deposit. Immobile element systematics of rhyolitic rocks from the Boundary deposit lack major differences in primary petrochemistry between hanging-wall and footwall strata. All rocks from the deposit area are subalkalic with transitional Zr/Y ratios (2.8–4.5), La/Sm ratios <1 (normalized to upper crust), and primitive mantle normalized signatures with slightly light rare earth element (LREE)-enriched patterns with flat heavy REE (HREE), and negative Nb, Ti, and Eu anomalies. These geochemical features, coupled with existing Nd-Pb isotope data, zircon inheritance patterns, and geologic information, are consistent with rhyolitic rocks at the Boundary deposit having formed by re-melting of arc basement, with continental crust (or recycled continental crust) present in the source region. It is likely that the deposit and its associated rhyolitic host rocks formed within a Cambrian continental(?) or pericontinental rifted arc along the margin of Ganderia, within the Iapetus Ocean.

The Role of Regional and Local Structure in a Late Ordovician (Edenian) Foreland Platform-to-Basin Succession Inboard of the Taconic Orogen, Central Canada

The Upper Ordovician (Edenian) Lindsay Formation of the Ottawa Embayment represents the final stage of carbonate platform development in the Taconic foreland periphery inboard of the northern Appalachian orogen. The succession overlies a narrow (~60 km) axis of a Neoproterozoic Laurentian rift extending across the Grenville orogen. The Lindsay Formation consists of a lower heavily bioturbated skeletal limestone that represents a warm-water shoal facies following an underlying outer ramp stratigraphy, and an upper division of renewed deep-water deposition with organic-rich shale and fossiliferous lime mudstone. Pyritic deep-water black shale of the westerly advancing Taconic foreland basin disconformably overlies this platform succession. Stratigraphic correlation through the central embayment identifies likely synsedimentary faults and seaward-directed erosion bounding the Lindsay Formation in a region of older Ordovician faults and a change in the lithotectonic character of the crystalline basement. The Late Ordovician shallowing and localization of structural/erosional features are interpreted to record a structural hinge: a local accommodation to, first, foreland periphery uplift, then rapid subsidence related to westerly diachronous foreland subsidence through the platform interior. Spatial association of structures of differing ages suggests that reactivation of inherited weakened crust influenced Late Ordovician sedimentary patterns.

Provenance variations in northern Appalachian Avalonia based on detrital zircon age patterns in Ediacaran and Cambrian sedimentary rocks, New Brunswick and Nova Scotia, Canada

Avalonia in the northern Appalachian orogen is generally assumed to have originated near the margin of Gondwana, although its position along that extensive margin and the timing of its separation remain disputed. U–Pb ages determined by laser ablation – inductively coupled plasma – mass spectrometry are reported here for detrital zircon from Avalonian clastic sedimentary units, three in southern New Brunswick, and one in southeastern Cape Breton Island, with depositional ages ranging from ca. 630 to ca. 505 Ma. Considered in combination with previously published detrital data sets from two other samples from southern New Brunswick deposited in a similar age range, the samples show a wide spectrum of ages from Ediacaran to Neoarchean (as old as 3.2 Ga). The dominant zircon populations in all six samples are Ediacaran–Cryogenian with ages between 770 and 540 Ma and can be matched with known magmatic events in Avalonia. Pre-Avalonian cycle detrital zircon dates in the samples are relatively sparse and do not reveal a consistent signature that might be used to place Avalonia in a unique paleogeographic position, although they are compatible with Amazonian provenance. An increase in ca. 2.2–1.9 Ga zircon dates in Early Cambrian through Early Ordovician samples suggests that sources for zircon of those ages became available in Avalonia by the end of the Ediacaran, possibly by motion of Avalonia to a different part of the Amazonian margin or to the West African craton.

The Dog Bay – Liberty Line and its significance for Silurian tectonics of the northern Appalachian orogen1This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.2Geological Survey of Canada Contribution 20100257.

The Dog Bay Line, a Silurian suture key to deciphering Appalachian accretionary history, was first recognized in Newfoundland. It marks where the Ordovician Tetagouch–Exploits ensimatic back-arc basin (TEB), which had opened within the leading peri-Gondwanan Gander terrane, finally closed. Here, we extrapolate this suture into New England, placing it between the Liberty–Orrington–Miramichi inliers (LOM) and the Merrimack–Fredericton trough (MFT). Southeastward, marine strata of the MFT overlie the TEB passive margin, exposed in the Ganderian St. Croix block, and display southeast-vergent structures transected by Acadian cleavage. They structurally underlie southeast-vergent thrusts at the base of the LOM. Northwestward, the LOM, Central Maine – Matapedia trough (CMMT), and Lower Silurian igneous rocks record elements of the upper plate trench–arc system, respectively, a subduction complex, forearc basin, and arc. The CMMT forearc received detritus both from the northwesterly arc region, and also from the Early Silurian-exhumed subduction complex. Minimal contrast in Silurian turbidites near the line may be due to sediment bypassing the subduction complex, and (or) a common provenance when the complex emerged above sea level. Salinic unconformities in the upper plate (arc–trench) reflect episodes of shortening, within an overall extensional setting that resulted in thinned, weakened lithosphere, and also final uplift accompanying latest Silurian slab breakoff. Silurian strata of the Coastal Volcanic Belt document a separate arc system built on Ganderia’s trailing edge, where northwest-directed subduction of a narrow seaway led to latest Silurian collision with buoyant, strong lithosphere of Avalonia’s passive margin, and the onset of Acadian typically dextral-oblique, northwest-vergent deformation.

Comparative evolution of the Iapetus and Rheic Oceans: A North America perspective

Differences in the origin and evolution of the Iapetan and Rheic oceans profoundly affected the development of the Ouachitan–Appalachian–Variscan orogen. The Iapetus Ocean was initiated in the Late Neoproterozoic–Early Cambrian by the separation of large continental landmasses (Laurentia, Baltica, Gondwana), whereas the Rheic Ocean originated in the Late Cambrian–Early Ordovician as the result of the separation of several ribbon continents, collectively termed peri-Gondwanan terranes, from the northern margin of Gondwana. Compared to the classic passive margin developed along the Laurentian margin of Iapetus, passive margin successions to the Rheic Ocean are more limited in extent. During convergence, both margins of the Iapetus Ocean developed western Pacific-type arc–back-arc systems, whereas subduction of the Rheic Ocean was primarily directed beneath Laurentia and was Andean in style. Closure of Iapetus involved a number of arc–back-arc accretionary events and had occurred by the Middle Silurian as a result of Laurentia–Baltica collision (to form Laurussia) and the accretion of the peri-Gondwanan terranes. Rheic Ocean closure occurred in the Carboniferous by way of continent–continent collision between Laurussia and Gondwana, with Gondwana on the lower plate. Within Iapetus, the formation of ophiolites occurred early in the orogenic cycle, and was followed soon after by obduction, HP–LT metamorphism and foreland-directed thrusting that is intimately related to accretionary events. These features dominate the northern Appalachian orogen where HT–LP metamorphism is attributed to post-accretionary slab break-off events. In contrast, HP–LT metamorphism within the Rheic Ocean reflects A-type subduction during Laurussia–Gondwana collision, and is rapidly succeeded by HT–LP metamorphism attributed to orogenic collapse. Ophiolites are rare and were formed either very early (Early Ordovician) or very late (Devonian–Carboniferous) in the evolution of the ocean. The Early Ordovician ophiolites were not obducted until the Carboniferous, at least 100 million years after their protoliths were formed. Rheic Ocean closure resulted in the formation of large-scale bivergent structures within the orogen, with symmetry changing on either side of the Rheic suture. Comparison of the evolution of the Iapetus and Rheic oceans suggests that differences in rift configuration, passive margin development, and subduction zone geometry within the two oceans profoundly influenced the subsequent style of orogenesis within the Ouachita–Appalachian–Variscan orogen.

Anisotropy of magnetic susceptibility fabrics in syntectonic plutons as tectonic strain markers: the example of the Canso pluton, Meguma Terrane, Nova Scotia

ABSTRACTThe anisotropy of magnetic susceptibility (AMS) is widely and routinely used to measure the preferred orientations of Fe-rich minerals in undeformed and weakly deformed granite plutons. The interpretation of the mapped AMS fabrics depends on rock-textural observations, on the map patterns of the fabrics in plutons, and on comparisons of the pluton fabrics to tectonic structures in the country rocks. The AMS may document emplacement-flow related fabrics, but the emplacement fabrics may be reworked or completely overprinted by rather weak tectonic strains of the magma mush or the cooling pluton, especially in syntectonic intrusions. The Late Devonian Canso granite pluton is an excellent example of overprinting of emplacement fabrics by weak tectonic strains. The Canso pluton was emplaced ca. 370 Ma along the boundary between the Meguma and Avalon tectonic terranes, in the northern Appalachian orogen. The AMS was mapped along two traverses that cross the pluton and that are perpendicular to the terrane boundary. Textural evidence suggests the rocks underwent very modest post-full crystallisation strains. The AMS records the dextral transcurrent shearing that occurred on the terrane boundary during emplacement and cooling of the Canso pluton, supporting interpretations that weakly deformed syntectonic granites can be used as indicators of regional bulk kinematics. AMS fabrics in Late Devonian granites of the Meguma Terrane suggest partitioning of the non-coaxial shearing into the terrane bounding fault, with pure-shear dominated deformation further from the fault. Numerical simulations suggest that the kinematics recorded by the fabrics in the Canso pluton was simple-shear, or transpression or transpression with small components of pure shear oriented perpendicular to the bounding shear zone.

Paleomagnetism of fluorite veins in the Devonian St. Lawrence granite, Newfoundland, Canada

Paleomagnetic results are reported from the Devonian St. Lawrence granite and the fluorite veins within the granite or its related porphyry dikes. Paleomagnetic analysis of 359 specimens from 29 sites shows that there is no statistically significant difference at 95% confidence between the site mean characteristic remanent magnetization (ChRM) directions carried by pyrrhotite and (or) magnetite of the granite and fluorite veins. No post-granite metamorphic event is known in the area. Thus, these concordant directions indicate that the granite and fluorite veins are coeval and retain a primary remanence. However, the mean ChRM direction of 25 vein and granite sites yields an Early Pennsylvanian paleopole of ∼316 Ma on the North American apparent polar wander path at 35.5°N, 129.2°E (A95 = 3.4°) that is younger than the 374 ± 2 Ma Devonian U–Pb zircon age for the granite. Two intersecting vertical vein sets indicate that the St. Lawrence granite has not been tilted since emplacement of the fluorite veins. Therefore, the difference between the measured and expected paleopoles is interpreted to result from a postemplacement counterclockwise rotation of ∼17° of the St. Lawrence granite region about a vertical axis. Analysis of Carboniferous paleopoles in the northern Appalachian orogen indicates the rotation at the St. Lawrence area is Late Devonian during the Acadian orogeny.

Altering Avalonia: oxygen isotopes and terrane distinction in the Appalachian peri-Gondwanan realmLaboratory for Stable Isotope Science (LSIS), The University of Western Ontario, Contribution 236.

Distinct 18O depletion is characteristic of a large majority of the 620–550 Ma felsic igneous rocks of Avalonia in the northern Appalachian orogen. Neoproterozoic rocks in the Boston Avalon terrane have the lowest δ18OWR values (≥–3.1‰), followed by the Mira terrane in Cape Breton Island and the Caledonia terrane in New Brunswick (≥–1.2‰), the Avalon terrane in Newfoundland (≥+2.8‰), and the Antigonish Highlands in Nova Scotia (≥+5.3‰). In contrast, this depletion of 18O is observed in very few of the Paleozoic felsic igneous rocks from these Avalonian terranes, and also in very few of the Neoproterozoic and Paleozoic felsic igneous rocks from the inboard Ganderian terranes. The low-18O character of the Neoproterozoic igneous rocks is related to regional pervasive, post-magmatic alteration by predominantly meteoric-hydrothermal fluids (δ18OH2O ∼–6‰ to –4‰) at 200–450 °C. The alteration likely occurred during late Neoproterozoic transtensional extension of Avalonia. Large-scale fluid infiltration and circulation within the Avalonian crust accompanied this extension with development of pull-apart sedimentary basins and extension-related magmatism that were the prelude to Cambrian submergence of Avalonia. This regional 18O depletion provides a geochemical fingerprint by which Avalonia can be distinguished from other peri-Gondwanan terranes. These data suggest that Avalonia existed as a composite terrane on the Gondwanan margin in the Neoproterozoic, separate from Ganderia.

Characteristics of Mineralization at the Main Zone of the Clarence Stream Gold Deposit, Southwestern New Brunswick, Canada: Evidence for an Intrusion-Related Gold System in the Northern Appalachian Orogen

Mineralization at the Main zone of the Clarence Stream gold deposit in southwestern New Brunswick occurs within a parallel series of quartz veins that occupy a NE-trending, steeply north-dip- ping, brittle-ductile shear zone related to the nearby terrane-bounding Sawyer Brook fault. This deforma- tion zone cuts volcaniclastic sedimentary and volcanic rocks of the Silurian Waweig Formation, and is intruded by mantle-derived East Branch Brook gabbroic dikes and a number of felsic dikes. Intermittent shear-zone reactivation during local magmatic activity induced heterogeneous deformation and alteration of the country rocks and dikes. The terminal phase of shearing is associated with narrow pegmatite-aplite dikes that grade laterally into granophyric granite, and into an auriferous quartz vein. Gold, aurostibite, and gold-antimony intergrowths, predominantly with vein quartz, are associated with a low ƒO2 sulfide mineral assemblage (pyrrhotite, arsenopyrite, and berthierite) that has a sulfur isotopic signature consistent with a magmatic source. Gold saturation and deposition occurred episodically between 300° and 360°C at low activity of sulfur, predominantly in response to pressure reduction during brittle failure combined with the effects of decreasing temperature and sulfidation reactions during fluid-wall-rock interaction. Geo - chronological studies demonstrate that the timing of the emplacement of the pegmatite-aplite dikes and associated auriferous veins overlap with that of the Early Devonian Magaguadavic granite. Moreover, the composition of the dikes is consistent with their derivation from this granite as late fractionates. The Main zone of the Clarence Stream deposit is therefore interpreted to be intrusion-related, with the Magaguad- avic granite being the ultimate source of the gold-bearing fluids. © 2008 Canadian Institute of Mining, Metallurgy and Petroleum. All rights reserved.

Nick Rast and the recognition of the Avalonian Arc

Recognition of the eastern (Avalonian) margin of the northern Appalachian orogen as a Late Precambrian microcontinental arc terrane, rather than the opposing passive margin of the Proto-Atlantic (Iapetus) Ocean to that of eastern Laurentia, constituted a fundamental advance in Appalachian geology that profoundly influenced subsequent models for the orogen's plate tectonic evolution. This advance was first clearly articulated by Nick Rast and his students in 1976, who, by correlating rocks of the Avalon Platform with those of the British Midlands, established the Avalonian volcanic belt as a Japan-like microcontinent. Contrary to contemporary views of the Avalon Platform, which favored an extensional, Basin and Range-like setting for its volcanism, Rast argued on the basis of this correlation that the association of Avalonian volcanism with compressional orogeny, widespread calc-alkaline plutonism and, in Angelsey, with blueschists and ophiolitic rocks, indicated a convergent plate margin setting. Rast further proposed that the Avalonian volcanic belt was ensialic, and was bordered to the northwest and southeast by Precambrian oceanic domains. Contemporary reconstructions of the Avalonian and Cadomian belts as fragments of a Cordilleran-like accretionary orogen that developed along an active margin of Neoproterozoic Gondwana owe their origin to these early ideas and, while far removed from the tectonic model that Rast envisaged, are a direct heritage of his recognition of the Avalonian volcanic belt as a microcontinental arc terrane.

Late Cretaceous reactivation of the Norumbega fault zone, Maine: Evidence from apatite fission-track ages

Apatite fission-track ages from opposite sides of the Norumbega fault zone in southern Maine reveal a significant Early Cretaceous time-temperature discontinuity across the structure. Ages west of the fault zone range from 113 to 89 Ma, while those to the east are significantly older, ranging from 159 to 140 Ma. Modeling of track length distributions reveals that at 120 Ma, an ∼55 °C difference in temperature existed between opposite sides of the fault zone currently exposed at the surface. This discontinuity is best explained by an episode of significant (to 2 km of net vertical separation) east-side-down vertical movement along the Norumbega fault zone in Late Cretaceous time. The modeling suggests contemporaneous cooling and that final movement along the fault zone occurred ca. 80 Ma. These results, when combined with earlier structural and thermochronological studies, suggest that faults mapped within the Norumbega fault zone were intermittently active from Middle Devonian to Late Cretaceous time, and that the structure is a fundamental zone of crustal weakness. The prolonged period of activity likely reflects stresses associated with late Paleozoic dextral transgression, early Mesozoic rifting associated with the initial opening of the North Atlantic Ocean basin, and processes associated with the late Mesozoic uplift and exhumation of the northern Appalachian orogen.

Deformation partitioning during transpression in response to Early Devonian oblique convergence, northern Appalachian orogen, USA

Transpressive deformation was distributed heterogeneously within the Central Maine belt shear zone system, which formed in response to Early Devonian oblique convergence during the Acadian orogeny in the northern Appalachians. ‘Straight’ belts are characterized by tight folds, S>L fabrics and sub-parallel form lines, and asymmetric structures that together indicate dextral–SE-side-up kinematics. In contrast, intervening zones between ‘straight’ belts are characterized by open folds and L≫S fabrics. Within both types of zone, metasedimentary rocks have fabrics defined by the same minerals at the same metamorphic grade, including a penetrative, moderately to steeply NE-plunging mineral lineation. Thus, we interpret accumulation of plastic deformation and regional metamorphic (re-) crystallization to have been synchronous across the Central Maine belt shear zone system. Discordance between inclusion trails in regionally developed porphyroblasts of garnet and staurolite and matrix fabrics in ‘straight’ belt rocks records shortening by tightening of folds and greater reorientation of matrix fabrics with respect to porphyroblasts. Kinematic partitioning of flow was responsible for the contrasting states of finite deformation recorded in the Central Maine belt shear zone system. Perturbations in the flow were caused by serially developed thrust-ramp anticlines in the stratigraphic succession immediately above the Avalon-like basement, at which décollement of the shear zone system was initially rooted. General shear deformation at the ramps involved strain softening with an enhanced component of noncoaxial flow. In contrast, deformation during extrusion in the intervening zones involved strain hardening with a greater component of coaxial flow. Part of the thickening stratigraphic succession exceeded T solidus, reflected by the occurrence of migmatites and granites. The latter were partly sourced from the underlying Avalon-like basement that was involved in the deformation and melting.